I. Field of the Invention
The present invention pertains phase change data storage devices having 2 or more bits of data per memory cell that are achieved by attaining multiple reflectivities. More particularly, the present invention pertains to an optical data storage devices having a single or multiple layer(s) of phase change medium that achieves multi-level reflectivity.
II. Description of the Background
Nonablative state changeable data storage systems, for example, phase change optical data storage systems, record information in a state changeable material that is switchable between at least two detectable states by the application of energy, such as, for example, laser light, projected optical beam energy, electrical energy, or thermal energy, thereto.
State changeable data storage materials are typically incorporated into a data storage device having a structure such that the data storage material is supported on a substrate and protected by encapsulants. In the case of optical data storage devices, the encapsulants include, for example, anti-ablation materials and layers, thermal insulation materials and layers, anti-reflection materials and layers, reflective layers, and chemical isolation layers. Moreover, various layers are used to perform one or more of these functions. For example, anti-reflection layers may also be used as anti-ablation layers and/or thermal insulating layers. The thickness of the layers, including the layer of state changeable data storage material, are engineered to minimize the energy necessary for effecting the state change and to optimize the high contrast ratio, high carrier to noise ratio, and high stability of the phase change materials.
The phase changeable material is a medium capable of being switched from one detectable state to another detectable state or states by the application of energy, such as projected beam energy, electrical energy, or thermal energy thereto. The detectable states of state changeable materials may differ in their morphology, surface topography, relative degree of order, relative degree of disorder, electrical properties, optical properties, including indices of refraction and reflectivity, or combinations of one or more of the foregoing. The state of the phase changeable material is detectable by properties such as, for example, the electrical conductivity, electrical resistivity, optical transmissivity, optical absorption, optical refraction, optical reflectivity, or combinations thereof. That is, the magnitude of the detectable property will vary in a substantially predictable manner as the phase changeable material changes state.
Phase change data storage devices are typically made by the deposition of individual layers onto a substrate. A number of techniques have been developed which include evaporative deposition, chemical vapor deposition, and/or plasma deposition. As used herein plasma deposition includes, but is not limited to, sputtering, glow discharge, plasma assisted chemical vapor deposition, microwave plasma vapor deposition, etc.
One example of phase change memory materials used in data devices include the Tellurium based alloys. The state change of Tellurium based alloys is a structural change evidenced by a change in a physical property, such as reflectivity. This effect is described, for example, in J. Feinleib, J. deNeufville, S. C. Moss, and S. R. Ovshinsky, “Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors,” Appl. Phys. Lett., Vol. 18(6), pages 254-257 (Mar. 15, 1971).
Tellurium based state changeable alloys, in general, are single or multi-phased systems where: (1) the ordering phenomena includes a nucleation and/or growth process (including both or either homogeneous and heterogeneous nucleations) to convert a system of disordered materials to a system of ordered and disordered materials; and (2) the vitrification phenomena include melting and rapid quenching of the phase changeable material to transform a system of disordered and ordered materials to a system of largely disordered materials. The above phase changes and separations occur over relatively small distances, with intimate interlocking of the phases and gross structural discrimination, and are highly sensitive to local variations in stoichiometry.
In chalcogenide type memory materials, the measures of performance include (1) the contrast ratio, that is, the difference in reflectivity of the states divided by the sums of the reflectivity of the states, and (2) the carrier to noise ratios of both (a) the “written” and (b) the “erased” states. The failure mode of the memory material as evidenced by the deterioration in the measures of performance with respect to the number of cycles. That is, failure may be evidenced by for example, a reduction in contrast ratio with increasing cycles, by a reduction in the written carrier to noise ratio or by an increase in the erased carrier-to-noise ratio.
In present day phase change memory devices it is customary to write or record onto an initial crystalline state by switching it to an amorphous state. The integrity of recording depends on the ability of the amorphous second state to resist spontaneous or accidental crystallization into the written state. The higher the glass transition temperature of the recording medium, the more likely the medium will resist undesirable crystallization when in the amorphous state, particularly when the recorded region consists of alternating regions of recorded crystalline spots and unaltered amorphous regions. Of course, the crystallization temperature must not be so high that recording sensitivity suffers significantly.
In addition to the write capability of phase change recording devices, it may be desirable that the memory material also be erasable. In a typical application, the phase change material is reversibly capable of changing from the amorphous to the crystalline state, and back again, repeatedly, such that memory medium may be recorded upon, erased, re-recorded upon, re-erased numerous times.
One recognized limitation of present day, phase change data storage devices is in the capability to store increasing amounts of data. One system, which has been reported to increase the data storage capacity of conventional memory technology, is multi-level recording. Multi-level recording offers the potential to produce more than two bytes of data per memory cell.
One way to obtain multi-level recording is with a single layer of memory material, which has more that two detectable states. See for example U.S. Pat. No. 5,335,219, entitled “Homogeneous Composition of Microcrystalline Semiconductor Material, Semiconductor Devices and Directly Overwritable Memory Elements Fabricated Therefrom, and Arrays Fabricated from the Memory Elements”, issued on Aug. 2, 1994 to Ovshinsky et. al., the disclosure of which is herein incorporated by reference.
However, problems associated with accurately writing and detecting multiple signal levels have limited commercial use of multi-level recording applications. These problems occur primarily from poor signal to noise or sigma-to-dynamic range capabilities of the phase change recording material. One attempt to address the problems of high relative noise and inaccurate signal reproduction involves error detection and correction schemes. Error detection and correction schemes are supposed to compensate for problems associated with writing accuracy and repeatability. Although these schemes can greatly compensate for write inaccuracy, error correction often uses otherwise employable storage capacity. Error correction also increases read and access times. Therefore, there is presently a need for an improved high density, multi-level phase change memory materials that can be accurately written to with numerous signal levels.
Multi-level recording can be produced by several different writing methods. One method, which will be designated as the fast cooling method, utilizes a single laser pulse. The pulse heats the chalcogenide alloy above the crystallization temperature, and after the laser pulse ends, the chalcogenide alloy rapidly cools to form a mixture of amorphous and crystalline phases over a given volume of memory material. The second method utilizes slow cooling and may be done with one, two or more laser pulses. When two laser pulses are used, a first laser pulse heats the chalcogenide alloy to a temperature above melting, and a second laser pulse, delivered a short period of time after the first laser pulse, alters the cooling rate causing partial recrystallization. Alternatively, the slow cooling method can be provided by a single laser pulse having multiple power levels. The laser pulse may be provided first with a high power (>12 mw) to melt the phase change material and then with a second lower power, such as medium power (5-9 mW) to reduce the cooling rate of recrystallization. Crystallization by the fast cooling method can be achieved easily at a wide range of speeds (1 to >15 m/s) and is a mechanism by which phase change optical recording occurs. However, overwriting is often unacceptable due to incomplete erasure of previous data. The slow cooling method, on the other hand, raises the temperature of the material above the melting point to aid in erasing or overwriting data. However, the mechanism by which this method works makes functioning at faster speeds (>2 m/s) difficult.